1. Field of the Invention
The present invention relates to an objective lens actuator for use in an apparatus for writing or reading information into or from a recording medium.
2. Statement of the Related Art
Generally, an information writing/reading apparatus employs a disk-shaped recording medium such as a compact disk, magneto-optical disk, etc. during operation. Such recording medium, however, has some degree of warpage and eccentricity and the apparatus is subjected to focussing deviations due to up-and-down motions due to warpage and is also subjected to tracking deviations due to eccentricity, etc of the recording medium. In order to correct such deviations, an objective lens actuator is arranged such that the objective lens is actuated along two axes. The direction of one axis is the direction of an optical axis (hereinafter referred to as "focussing direction F") perpendicular to the surface of a recording medium and the direction of the other axis is a radial direction (hereinafter, "tracking direction T") parallel to the surface of a recording medium. In general, however, the region over which the objective lens is moved in the tracking direction T is smaller than an information writing/reading region of the recording medium. For this reason, in order to effect writing and reading of information at a given position on the recording medium, the objective lens actuator as a whole is driven for recording medium accessing by a motor in the radial direction (the same direction as the tracking direction) of the recording medium. In addition, there has recently been an increasing demand in the art for reduction in size and thickness, enhancement in performance, and shortening of access time. Accordingly, it is also necessary that the objective lens actuator as a whole be made small in size and weight and be capable of operating with a high degree of efficiency.
Conventionally, an objective lens actuator of this type has a structure as shown in FIG. 6, which is disclosed in, for example, Japanese Patent Unexamined Publication No. 57-210457.
Namely, a magnet 26, first magnetic yokes 27a, 27b and second magnetic yokes 28a, 28b are provided on a base 9, to thereby form a magnetic circuit. This magnetic circuit generates magnetic fluxes. On the other hand, a focussing coil 6 is mounted on a lens holder 2 provided with an objective lens 1 and permits the passage therethrough of an electric current. A drive force is produced due to the actions of the above magnetic fluxes and electric current. Actuation of the objective lens 1 in the focussing direction F is effected by sliding motion of the lens holder 2 along a shaft 3 via a bearing 23, which sliding motion is caused to take place due to the action of said drive force. Further, tracking coils 5a to 5d are mounted on the lens holder 2, and each has coil surface 12d through which an electric current is allowed to pass. A drive force is produced due to the actions of the above-mentioned magnetic fluxes and this electric current. Actuation of the objective lens 1 in the tracking direction T is effected by turning motion of the lens holder 2 about the shaft 3 via the bearing, which turning motion is caused to occur due to said drive force. In addition, the objective lens actuator as a whole is driven for access by a motor in the access direction Ac (the same direction as the tracking direction T) on the recording medium.
In the objective lens actuator having a construction as shown in FIG. 6, however, the coil 6 for use in focussing is wound around the lens holder 2. For this reason, the magnetic fluxes, which are generated from the magnetic yokes 27a, 27b and the second magnetic yokes 28a, 28b, are arranged so as to be generated toward a center of the shaft 3. Accordingly, a face 12c of each of the tracking coils 5a to 5d must be located outside the magnetic circuit. In addition, the objective lens actuator is a planar type actuator. When it is desired to achieve thinning of the actuator as a whole, therefore, it is impossible to employ large effective coil length for obtaining a drive force acting in the tracking direction T. Thus, the use efficiency of the coil becomes low. Furthermore, positioning and fixing of the tracking coils onto the focussing coil 6 becomes difficult.